Stents are well known medical devices that are used for maintaining the patency of a large variety of vessels of the human body. A more frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, and some current stent designs such as the CORDIS BX Velocity® stent, Cordis Corporation, Miami Lakes, Fla., have the required flexibility and radial rigidity to provide an excellent clinical result, they are not always clearly seen under standard fluoroscopy.
Many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the arterial wall. A single strut member can be thought of as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current stent designs such as the BX Velocity® stent, these sets of strut members are formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. Although a stent with uniform width of the strut members will function, if the width is increased to add strength or radiopacity, the sets of strut members will experience increased strain upon expansion. High strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Existing highly radiopaque stents, such as the gold plated NIROYAL stent sold by Boston Scientific, Inc., Natick Mass., can obscure the inside of the vessel due to the high radiopacity over the entire length of the stent. The BeStent sold by Medtronic, Inc., Minneapolis Minn., has small gold markers at the ends of the stent. Those markers only mark an end point without allowing visualization of the entire end set of strut members.
Fischell et al, in U.S. Pat. No. 6,086,604, discloses a stent with the end sets of strut members being gold plated. Such a stent would have ideal radiopacity but may be subject to the corrosive effects incurred through placement of dissimilar metals in an electrolytic solution such as blood. There has also been significant evidence that gold is a poor surface material for stents because it may increase the risk of subacute thrombosis or restenosis. Further, Fischell et al, in U.S. Pat. No. 5,697,971 discloses in its FIG. 7, a stainless steel stent with increased width diagonal sections in all the circumferential sets of strut members.